In a surprise addition to the witness line-up, NSW Premier Barry O'Farrell has been called to give evidence in a corruption inquiry into a company linked to the Obeid family.

Counsel assisting the Independent Commission Against Corruption, Geoffrey Watson, SC, said Mr O'Farrell would give evidence on Tuesday in the long-running inquiry into infrastructure company Australian Water Holdings.

"From the outset I was in two minds as to whether or not Mr O'Farrell should be called.

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"There have been things which have developed which make me think he should be called, or that he must be called."

The inquiry entered its fifth week on Monday and is expected to conclude within days.

Mr O'Farrell could not give evidence last week because he was in China on a trade mission.

Greg Pearce, the former finance minister in the O'Farrell government, gave evidence last week of a "cosy" meeting between the then chief executive of Australian Water, Nick Di Girolamo, and Mr O'Farrell.

Mr Pearce said he felt "like a schoolboy being called in to explain to the headmaster why he hadn't done his homework" when he walked into the meeting on May 27, 2011.

The commission is investigating allegations that the family of corrupt former Labor minister Eddie Obeid were "secret stakeholders" in Australian Water and stood to make up to $60 million from a proposed public-private partnership.

Mr Di Girolamo, a prominent Liberal Party fund-raiser and associate of the Obeid family, lobbied the O'Farrell government over the proposal.

The company also allegedly billed the state-owned Sydney Water secretly for millions of dollars in "administrative" expenses - including limousines and donations to the NSW Liberal Party - under a contract to supply water and sewerage infrastructure.

Within minutes of Mr Di Girolamo entering the witness box, Mr Watson had accused him of attempting to "milk it and get every cent you could get out of Sydney Water Corporation".

"I reject that, Mr Watson," Mr Di Girolamo said.

Australian Water became one of the biggest donors to the NSW Liberal Party in the months before the March 2011 election that swept Labor from power and installed the Barry O'Farrell-led Coalition.

In a statement, Mr O'Farrell said: "As I respect the ICAC and its role and process, I will not pre-empt any of the matters or any of the questions it may put to me.